Rates of women who are opting for preventive mastectomies, such as Angeline Jolie, have increased by an estimated 50 percent in recent years, experts say. But many doctors are puzzled because the operation doesn't carry a 100 percent guarantee, it's major surgery -- and women have other options, from a once-a-day pill to careful monitoring.

The bacterium Staphylococcusaureus may cause anything from minor skin lesions to life-threatening pneumonia, meningitis and toxic shock syndrome. In the past 15 years or so, it has become one of the most common germs to get spread around hospitals, with some 500,000 patients in U.S. hospitals contracting a "staph" infection each year.

Disturbingly, one strain of the microbe, dubbed MRSA , has evolved into a widespread "superbug" that has become resistant to many commonly used antibiotics. This variant is nearly three times more lethal than less drug-resistant strains.

Instead of attacking the bacterium with new antibiotics that it could evolve a resistance against over time, immunologist Domenico Iannelli at the University of Naples Federico II in Italy and his colleagues tried using bacteria-killing viruses known as bacteriophages or phages. These do not infect humans.

The researchers identified a phage that preys on Staphylococcus aureus. They next experimented with the virus on mice given either lethal or non-lethal doses of the bacterium, including MRSA strains.

Iannelli and his colleagues found phage therapy could rescue 97 percent of mice given lethal doses from death. When given to mice with nonlethal doses, the virus fully eliminated the infections.

Although bacteria eventually develop resistance against phages as they do against antibiotics, "phage resistance is a rare event, rarer than antibiotic resistance," Iannelli told LiveScience. "New phages can be rapidly isolated from nature, while new antibiotics are very slow and costly to develop."

A complaint often leveled against phage therapy is that phages typically each only attack very specific targets.

"If you use just a single phage, it's unlikely to kill all MRSA strains," said Alexander Sulakvelidze, chief scientist at biotechnology company Intralytix in Baltimore, who did not participate in this study. "The solution is to use a cocktail of phages, each targeting different strains. Phage therapy could provide a good, necessary alternative to antibiotics, another weapon against bacteria."

The researchers suggest the phage could help control human staph infections, taking phage therapy "closer to the patient's bed," Iannelli said.

"I was very pleased to see work of this scientific caliber," Sulakvelidze said. "I hope to see their research benefit people in the not-too-distant future."

Iannelli and his colleagues detailed their findings in the August issue of the journal Antimicrobial Agents and Chemotherapy.